The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
In a conventional converter, it is required to put a resistor in a power supply line to restrict an inrush current due to a rectification by a diode bridge. In such converter, however, steep and high current initially flows when an AC power source is turned on in a state in which a condenser is not charged. In order to restrain such current, it is required to increase the value of resistance to be inserted in the power supply line. However, this causes a longer charging time. In order to avoid these drawbacks, a charge control using a thyristor can be considered. In a conventional method, a firing timing of a thyristor is determined by measuring a cycle of an AC power supply and estimating the phase. In another control method (e.g., Japanese Unexamined Laid-open Patent Publication No. H04-26372), a firing angle of a thyristor is calculated by detecting an AC power supply phase and the thyristor is fired at a certain voltage difference by comparing the AC voltage and the capacitor voltage at the time of firing the thyristor.
As explained above, in a conventional thyristor converter, the capacitor was charged while controlling the firing angle based on the detection of the AC power supply phase and the comparison of the AC voltage and the capacitor voltage.
In a conventional thyristor converter, since the steps of detecting the phase detection using the voltage detection as it is and the voltage difference of the AC voltage and the capacitor voltage and charging a capacitor while controlling the firing angle have been employed, there was such problem that the thyristor was fired at an erroneous timing since no constant voltage difference could be obtained due to a failure of accurate power supply phase detection and voltage detection in cases where the detected power supply voltage had distortion. There also were problems that charging time was dispersed due to the connected power supply impedance and the capacitance of the capacitor and that the charging time increased due to the single phase thyristor firing control.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.